Scheduling, Precedence Diagramming and the Critical Path

Scheduling, Precedence Diagramming and the Critical Path (#23 in the Hut Introduction to Project Management)
By JISC infoNet

Planning the project activities helps us to work out and clearly communicate what we need to do, who needs to do it, and in what order. One of the big reasons that some projects fail is that the list of activities to be undertaken is incomplete or isn’t collated into a coherent plan. By first thinking of the products that are needed, it can make this task easier:

• Produce the Product Breakdown and agree this as the key milestones and phases with the Sponsor/Project Board.

• Agree the major tasks required with the project team. Writing each task on a Post-It note can help. This leads to what is sometimes called the Work Breakdown Structure.

• People with the most knowledge and/or experience should then compile the detailed activity list. i.e. break down each task into its constituent activities e.g.:

Project	Produce a new course brochure
Major Task	Check the proofs
Activity List	Check spelling, check layout, check colours, etc.

• Establish where any dependencies exist between activities.

Many of the project planning software packages available use the Precedence Diagramming Method. This method plots the tasks to be completed and connects them with arrows that show the dependencies.

Mandatory dependencies are inherent in the work or process e.g. when constructing a new building, building the walls is dependent on laying the foundations. Discretionary dependencies are those defined by the project manager and their team. They should be defined based on best practice or previous experience within the particular area. Once the dependencies are agreed they can be mapped into a Precedence Diagram (on PC, on paper, or using post-it notes).

When drawing the precedence diagram the project team needs to decide:

• Which tasks can only be completed after another task.
• Which tasks can be done at the same time.
• Which tasks don’t depend on other tasks at all (e.g. project review meetings).

It can be useful to work backwards when compiling the Precedence Diagram and ask yourself what do we need to have done immediately before this task?

When compiling the diagram you normally represent a task as a box, and link tasks with arrows to show any precedent. There are no loops and at any stage, all preceding tasks must be complete before the following task can begin.

The Precedence Diagram

It is possible to draw these from left to right with the final task on the right, or from top to bottom as many people prefer to work this way. As long as your approach is consistent and understandable by the team, there is no reason why you cannot choose your preferred method.

Once the project activities and any precedences have been identified and agreed, you will need to produce a schedule. As well as scheduling activities you may also need to schedule people, money, materials and equipment.

To schedule in detail you will have to establish the following information about each activity:

• The duration (DUR) of each activity – how long it will take to complete.
• The earliest start time (EST) – the earliest an activity can start without interfering with the completion of any preceding activity.
• The latest start time (LST) – the latest an activity can start without interfering with the start of any subsequent activity.
• The earliest finish time (EFT) – the earliest an activity can finish.
• The latest finish time (LFT) – the latest an activity can finish without interfering with the start of any subsequent activity.
• The “float” time of an activity – the time available to perform the activity less the time needed i.e. time available minus activity duration.

The critical activities are those with zero float. i.e. for a critical activity, EST = LST .

A critical path appears on any precedence diagram (or Gantt Chart) and links tasks which have no float. You should therefore be able to trace a critical path through your project from start to finish.

• Estimate the time required to complete each project activity.
• Calculate the time available to complete each activity.
• Calculate each activity’s float, i.e. (LFT – [EST + DUR]).
• Identify the critical path (zero float activities).
• Calculate the total project duration.
• Agree the resources needed and their availability with the Executive/Project Board. Adjust resources and/or schedule if necessary.
• Agree the schedule with the project team and other stakeholders.
• Prepare and publish the project schedule. Using a Gantt chart is one way.

Many times the project duration is not the result of the project manager’s estimates or even anyone else’s calculations. It can be a deadline set by funders of the project or senior management as a wished for end date, or as a whim. It is only by doing these calculations that you can ascertain whether or not it is feasible to complete the project within the deadline.

A standard way of scheduling is to add dates and durations to each task box in the precedence diagram that we have seen already.

JISC infoNet aims to be the UK’s leading advisory service for managers in the post-compulsory education sector promoting the effective strategic planning, implementation and management of information and learning technology.